Several small steps taken in thin kickers with kick=0.

[William Shields]

Several steps are taken when passing through thin kickers with kick=0. Any simulations end successfully, but are slower.

Run with verbose_G4stepping=2 and you'll see the large number of steps in the thin elements.

Bug seen in develop, unsure about other branches.

[Laurie Nevay]

Fixed with commit https://bitbucket.org/jairhul/bdsim/commits/b69aabd42946c067f421fb3c91727fdb043ebd14